[Question] GSXR 750 motor installed in Seadoo jetboat
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GSXR 750 motor installed in Seadoo jetboat

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    Default GSXR 750 motor installed in Seadoo jetboat

    I know....CRAZY! I have a 2004 Seadoo Sportster with a 2006 Suzuki GSXR 750 motor installed with tach in dash. Here's my question....Im trying to figure out the "gear reduction" of the current setup. It's taking me 4000 RPM to make the boat have forward thrust and at 8000 RPM the motor is screaming and boat is on plane and going fast. I have almost no reverse thrust unless I rev it to like 6000RPM. I had a cavitation problem but fixed that.
    Trying to figure out if motor is spinning at 5000 RPM what is the impeller spinning at? I know there is a formula to figure it out along with RPM's but I have no clue. Do i count the teeth on the sprockets?
    Any assistance is greatly appreciated. Here is a photo of the sprockets and chains.
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    Default Here are some photos of boat ect....see clutch pedal and shifter?

    Crazy
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    Senior Member 74Bonneville's Avatar
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    This is an interesting project you have going here. Growing up I first took apart bicycles then motorcycles and so on. I had several dirt bikes and finished with a Kawasaki street bike. The sprockets are what you're going to have to look at. When I wanted to be quicker I used a larger rear sprocket. You want the opposite. You need to get the impeller turning faster so the smaller the better. Our jet boats connect the crank to the impeller so they spin at the same rate. Your set up doesn't look like that would work. I would go smaller until you lose to much bottom end. You'll have to experiment until you find the right gear ratio for you. Besides all of this you have a transmission with how many gears? 6?


    Good luck


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    Quote Originally Posted by 74Bonneville View Post
    This is an interesting project you have going here. Growing up I first took apart bicycles then motorcycles and so on. I had several dirt bikes and finished with a Kawasaki street bike. The sprockets are what you're going to have to look at. When I wanted to be quicker I used a larger rear sprocket. You want the opposite. You need to get the impeller turning faster so the smaller the better. Our jet boats connect the crank to the impeller so they spin at the same rate. Your set up doesn't look like that would work. I would go smaller until you lose to much bottom end. You'll have to experiment until you find the right gear ratio for you. Besides all of this you have a transmission with how many gears? 6?


    Good luck

    Yes, 6 gears. I just automatically shifted to 6 gear and left it there. New wear ring and resealed entire pump with new rings and gaskets. Have to figure out thrust so I can get in impeller bent.....

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    Or Seth, either one Budweiser's Avatar
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    Total of four sprockets in your set up right? How many teeth per sprocket?

    Starting at the engine, then the engine side of the idler (or whatever you wanna call it) shaft, then the pump side of the idler shaft, then the pump.

    And lastly the drive ratio for 6th gear...


    Cool project BTW

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    Hoodys and swim trunks Elkidminoltu's Avatar
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    According to the following link, Specifications: 2004 Suzuki GSXR750, 6th gear is transmission ration is 5.551:1, and final trans gear teeth ratio of 1.181:1. If I'm reading this correctly, you're going to want to move to more of a 1:1 or 0.90:1 ratio to get some overspeed (in relation to engine RPM) out of the impeller.

    So, if your engine sprocket and jackshaft input are the same sprocket size, then they are 1:1. To increase the pump speed but maintain the current engine speed you'll have to put a larger sprocket on the jackshaft output and smaller gear on the impeller shaft, like a pedal bike. This would start to move you closer to an "overdrive" of sorts.

    The downside is you start losing the mechanical advantage of the undergearing (see torque) and will require more HP to turn the shafts. In theory, you could overgear it in 6th and use the lower trans gears to get it on plane and then shift up once you're rolling.

    Good luck.
    Last edited by Elkidminoltu; 04-15-2015 at 12:42 PM.

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    Hoodys and swim trunks Elkidminoltu's Avatar
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    More info: Stock Seadoo's had ran around 5700 RPM redline, made 85hp and ran about 50mph. You GSX-R motor can do 14k RPM and makes ~155hp, 65ft/lbs of torque so it should push it to at least 50mph when geared correctly.

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    wow...thats a ton of GREAT info!! Going to count teeth.

    With this bike motor in boat does it matter which gear I'm in? I would assume just go to 5th or 6th gear so you can get into the higher rpm's without over working the motor. Is the shaft spinning with more torque at the lower gears? How's that? Im thinking that the trans is "useless" in this application cause all I want to do is spin the impeller at a higher rate of rotation, correct?
    Eventually the impeller and pump can handle only so much..... The trans is 100% needed in a street bike application but not in boat...would that be a true statement?? I'm in waaay over my head with this one.

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    Hoodys and swim trunks Elkidminoltu's Avatar
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    As old as this video is, the physics haven't changed and it's a good explanation of levers and gears to help you understand how your transmission delivers torque.


    The stock setup in that boat was a motor tied directly to the impeller with no gear reduction so it was a 1:1 ratio for engine RPM to impeller RPM. In the case of your setup, you have a transmission output (a), jackshaft input (b), jackshaft output (c) and impeller shaft input (d).

    On your engine, 1st gear is 2.785:1 meaning the transmission output shaft spins ~1/3 the speed of the engine crank in 1st; where-as 6th gear is 1.181:1 which means the transmission output shaft is spinning ~1/1.2 of the crank speed. So if your motor is spinning 3000 RPM in 6th gear, the trans output shaft is spinning 2727 RPM. That's good for torque, but bad for shaft speed.

    Now if we add in the gears, we can bring those ratios down in difference and get you closer to 1:1. If you have 17 teeth on the transmission output shaft (a), and 20 teeth at the jackshaft (b), then your ratio is 1.176:1 for output shaft speed (b). Using the 2727 from above, your jackshaft speed is now 2318 RPM. Again, wrong direction for shaft speed. To increase speed, you need a bigger transmission shaft gear (a) and smaller jackshaft gear (b). BUT, since you are restricted to the size you can go for transmission output gear size, we can use the last two gears to get your gains (c & d).

    It looks like on your setup that your transmission output shaft and front jackshaft sprocket have the same number of teeth, please confirm. Let's assume they are and the ratio is 1:1. Bring back to the 2727 RPM at point b and now lets see what that means to gear c. Jackshaft RPM is 2727, the gear C is 25 teeth and Gear D is 16 teeth. That ratio is now 16/25 (rear/front) or 0.64:1. Divide 2727/0.64 and we see 4261 RPM. This means after the gearing that 3000 engine RPM = 4261 RPM at the impeller.

    Of course this is all theoretical until you count teeth and report back to us.

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    Main drive sprocket has 17 teeth to 20 teeth.
    Drive shaft sprocket has 19 teeth to 30 teeth.

    Picture will assist..
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  13. #11
    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by CollinCountyCab View Post
    Main drive sprocket has 17 teeth to 20 teeth.
    Drive shaft sprocket has 19 teeth to 30 teeth.

    Picture will assist..
    I see 17 to 20 being a 85%
    I see 30 to 19 being a 1.58%
    so overall would be a 1.34% over drive

    so if you change the 17 for an 18 you be at 1.42% or for a 19 would be 1.50%
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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    whoooooo.......Im not looking to change sprockets!! I'm trying to understand what's happening at the drive shaft (RPMS) when my tach/motor is spinning at 5000RPM.

    Need to get the right impeller pitch to utilize the power....

    Thanks

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    Default xhaust

    Be careful with that hot exhaust inside an enclosed compartment too.

  16. #14
    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by CollinCountyCab View Post
    whoooooo.......Im not looking to change sprockets!! I'm trying to understand what's happening at the drive shaft (RPMS) when my tach/motor is spinning at 5000RPM.

    Need to get the right impeller pitch to utilize the power....

    Thanks
    Then 5000 x 1.34 or 6700 rpm at the pump.
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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